The present invention relates generally to acceleration sensors and more specifically to acceleration sensors of the type adapted for use in an automotive vehicle equipped with an automatic occupant restraint device such as an air bag.
In the design of passive occupant restraint systems for modern passenger automobiles, it has been found desirable to place a number of acceleration sensors at selected locations on the body of a vehicle which electrically interconnect a source of electrical power and the passive occupant restraint system. For example, air bag restraint systems often employ an electrically operated igniter for activating a stored dry chemical for producing inflating gas for the air bag. Acceleration sensors are used to actuate the igniter.
The known acceleration sensors utilized for electrical activation of occupant restraint systems employ an acceleration sensing mass carried in a housing and preloaded to a rest position against inadvertent actuation and having its motion toward a position effecting the desired actuation damped in some fashion.
U.S. Pat. Nos. 3,974,350 to Breed and 4,097,699 to Larson are exemplary of such sensors, both including a gas damped mass moving against a mechanical spring load to effect switch actuation. U.S. Pat. No. 4,329,549 to Breed discloses a similar sensor in which a permanent magnet provides the preload force to the mass in a manner functionally similar to the springs of the previously mentioned patents, but since the mass moves away from the magnet during actuation, preloading force decreases with movement of the mass, which has been found to provide a desirable advantage for some vehicle acceleration sensing applications over the function of the spring-loaded mass devices previously used.
A disadvantage of the prior art sensors has been that while the sensors are functionally acceptable, their cost of manufacture has been relatively high. Difficulties in closely controlling peripheral clearances between the mass and the housing have created some of the manufacturability problems.
A co-pending application of applicant, U.S. Ser. No. 137,637, now U.S. Pat. No. 4,816,627, assigned to the assignee of the present invention, discloses an alternative design for a magnetically biased gas damped acceleration sensor, but it, along with sensors such as that disclosed in U.S. Pat. No. 4,329,549 to Breed, suffers from the additional disadvantage of relatively high weight because of the use of the permanent magnet as a biasing device.